1. Field of the Invention
The invention relates to the abatement of sulfur dioxide (SO.sub.2) from oxygen-containing gas streams and is particularly useful for the removal of SO.sub.2 from sulfuric acid plant effluent gases.
2. Description of the Prior Art
As a result of the current world-wide concern for our ecology, considerable emphasis has been laid upon protecting our environment from excessive or harmful pollution by industrial wastes of all kinds. Because of the economic importance of so many processes and operations, both domestic and industrial, which necessarily utilize substantial amounts of sulfur, one of the areas of major concern has been pollution of the air with sulfur-containing gases. Chief among these are industrial steam boilers, ore smelters and sulfuric acid plants, all of which produce substantial volumes of gases containing sulfur compounds, particularly SO.sub.2.
Because of the economic size of the problem, sulfur removal technology has received increasing technical attention and a wide variety of removal processes have been suggested. These may be categorized as follows:
1. Dry processes in which SO.sub.2 is absorbed by contact with solid metal oxides such as lime and limestone;
2. Dry processes in which SO.sub.2 is absorbed by contact with molten salt or activated carbon;
3. Wet processes in which the SO.sub.2 is absorbed by scrubbing the gas with an aqueous solution or slurry of solid absorbent material. Principal absorbents include the alkalis (sodium and ammonium hydroxide), alkaline earths (calcium and magnesium oxides) and weak sulfuric acid; and
4. Catalytic oxidation processes in which the SO.sub.2 -containing gas cleaned of dust is passed over a solid oxidation catalyst at high temperature to produce SO.sub.3 which is condensed with moisture to form concentrated H.sub.2 SO.sub.4.
A still further process involves treating the SO.sub.2 with an aqueous solution of hydrogen peroxide or other peroxidic acids to oxidize the SO.sub.2 to SO.sub.3 which is then absorbed. Furthermore, such processes can be combined, e.g. the catalytic oxidation of SO.sub.2 followed by treatment with peroxidic compounds to obtain quite complete SO.sub.2 abatement.
While such processes are useful in many situations, they are not practical in many instances for a variety of reasons. For example, the heterogeneous catalytic processes do not achieve adequate removal at economic flow rates. The dry processes may present serious solids disposal problems or serious regeneration problems. The dispersed or dissolved absorbent may present similar problems of liquid solution disposal. Any of the processes may present problems of economics at high abatement levels. Thus, there is considerable incentive for the development of SO.sub.2 abatement processes having improved economics at high standards of SO.sub.2 removal.